An Ottendorf cypher or a book cypher is a cypher written with three numbers and the key is a specific document, usually a book (the bible or a dictionary are often used). The first letter refers to the page number, the second to the line number, and the third to either the number of the letter or word in that line. If it is the word, then it is technically a code.
The book used by both parties needs to be exactly the same, even the same edition.
It's best not to use the same number sequence for the same letter or word. If "76-26-16" is an e, don't use it for all of the es, because a third party could see the repetition of that sequence and guess what letter it is (there are plenty of es to choose from!).
In popular culture, it has been used in National Treasure (2004) with Nicholas Cage. The cypher was on the back of the Declaration of Independence and the key was the Silence Dogood letters. It also appears in quite a few other books and movies.
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